New docs for WS-Federation auth

[Tratcher]

1. Write a short paragraph describing what the topic will cover.
   WsFederation is a new authentication handler that has been ported from Microsoft.Owin / ASP.NET 4.5. It is being released out of band against ASP.NET Core 2.0.0 dependencies. See [Announcement] WsFederation 2.0.0-preview1 out of band release aspnet/Security#1473.

2. Write an outline of the proposed topic contents.
   There should be a walk through similar to these: https://docs.microsoft.com/en-us/aspnet/core/security/authentication/social/

3. Where would you put the topic in the Table of Contents.
   https://docs.microsoft.com/en-us/aspnet/core/security/authentication/

[KoalaBear84]

Has this any relation to SAML 2.0? That's what I'm currently waiting for to be able to use .NET Core 2.0. Thanks!

[Tratcher]

Like these? They're included.
https://github.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet/blob/master/src/System.IdentityModel.Tokens.Saml/Saml2SecurityTokenHandler.cs

[KoalaBear84]

Thanks, I will look into it! :)

[Rick-Anderson]

@scottaddie can you schedule this in the next 3 sprints?

[scottaddie]

@Rick-Anderson No problem. I should have the bandwidth to tackle this in Sprint 129.